Antifuses are commonly used in large field programmable gate arrays. Typically, in a gate array to be programmed, all such antifuses are initially in a non-conducting state. Certain antifuses are chosen to become conductors, while others remain nonconductors. It is of course important to the proper operation of the programmed device that those antifuses chosen to remain non-conductive function in that way. With a field programmable gate array generally containing many hundred of thousands of antifuses, it is difficult to achieve this with a typical prior art type of antifuse, which will now be described.
FIGS. 1 through 4 show the fabrication of a typical prior art antifuse 10. As shown therein (FIG. 1), initially, a metal electrode 12 is provided over a silicon dioxide base 14. Next, an oxide layer 16 is deposited over the metal electrode 12. After application and patterning of a layer of photoresist, the oxide layer 16 is etched to provide an opening therein down to the electrode 12, to expose surface 24 of electrode 12. After removal of the patterned resist, a layer of antifuse material 18 such as amorphous silicon is deposited over the resulting structure (FIG. 2), so as to be in contact with a portion of the electrode 12. Resist is applied to the antifuse layer 18, and is patterned so that upon etching of the layer 18, such antifuse material takes the shape shown in FIG. 3.
After removal of such resist, a layer of metal 20, forming the second electrode, is deposited and patterned as shown in FIG. 3.
If has been found that the etching step of the oxide layer 16 down to the electrode 12 can cause yield problems, as now described, and with reference to FIG. 4, an enlarged view of a portion of FIG. 3.
As the etching of the oxide layer 16 takes place, it is common that a sharp corner is not formed in the areas 22 where the oxide 16 is adjacent the electrode 12. Rather, it will sometimes happen that resist residue and/or irregular oxide 23 remain in corners at areas 22 when etching is completed, i.e., when the electrode surface 24 is exposed. This flaw in the structure can cause a leakage path from the one electrode to the other, resulting in an antifuse which should be a nonconductor acting as a conductor. The problem lies in the unevenness of this etching in such a small device.
While such a flaw is relatively uncommon, it will be understood that each antifuse of the many hundreds of thousands thereof must operate properly.
Furthermore, because of the relatively large contact area between the antifuse layer 18 and the electrode 12 at surface 24, the antifuse 10 exhibits a high capacitance, resulting in degradation of performance of the device. Additionally, such an antifuse 10 takes up a substantial area as measured laterally. Lastly, the step coverage of metal 20 can be very poor when the aspect ratio of the contact opening is very high, (i.e., deep, small contacts).